The following description of the background of the invention is provided simply as an aid in understanding the invention and is not admitted to describe or constitute prior art to the invention.
During the last ten years, considerable advances have been made in the development of high-performance, or more recently ultra-high-performance, concretes with Portland cement. Ultra high performance concrete (UHPC) represents a major development step over high performance concrete (HPC), through the achievement of very high strength and very low permeability. Typically, UHPC's compressive strength varies from about 120 to 400 MPa, its tensile strength varies from about 10 to 30 MPa, and its modulus of elasticity is in the range of about 60 to 100 GPa.
UHPC benefits from being a “minimum defect” material—a material with a minimum amount of defects such as micro-cracks and interconnected pores with a maximum packing density. One approach to minimizing defects is the Macro Defect Free (MDF) approach, which uses polymers to fill in pores in the concrete matrix. The process required to manufacture MDF concretes is very demanding, and includes laminating and pressing. MDF concretes are susceptible to water damage, have a large amount of creep, and are very fragile. Another approach to minimizing defects is the Densified with Small Particles (DSP) approach, which uses high amounts of superplasticizer and silica fume in the concrete mix. DSP concretes must either use extremely hard coarse aggregates or eliminate them entirely in order to prevent the aggregates from being the weakest component of the mix. DSP concretes do not require the extreme manufacturing conditions that MDF concretes do, but DSP concretes have a much lower tensile strength. Addition of steel fibers has been considered to improve the ductility of DSP concrete.
Principles employed in conventional UHPC include improved homogeneity through elimination of coarse aggregate; enhanced packing density by optimization of the granular mixture through a wide distribution of powder size classes; improved matrix properties by the addition of a pozzolanic admixture such as silica fume; improved matrix properties by reducing water/binder ratio; enhanced ductility through inclusion of small steel fibers; and enhanced mechanical performance through post-set heat-treatment (90-150° C.) to transform amorphous hydrates into crystalline products, making an improved microstructure (tobermorite, xonotlite) possible.
Several types of UHPC have been developed in different countries and by different manufacturers. The main difference between the various types of UHPC is the type and amount of fibers used. The four main types of UHPC are Ceracem/BSI, compact reinforced composites (CRC), multi-scale cement composite (MSCC), and reactive powder concrete (RPC). RPC is the most commonly available UHPC and one such product is currently marketed under the name Ductual® by Lafarge, Bouygues and Rhodia.
RPC concrete mixes usually contain fine sand (150-600 μm), Portland cement (<100 μm), silica fume (0.1-0.2 μm), crushed quartz (5-30 μm), short fibers, superplasticizer, and water. A typical RPC concrete mix has about 38.8% sand, 22.7% Portland cement, 10.6% silica fume, 8.1% crushed quartz, 2.0% steel fiber or organic fiber, 1.4% superplasticizer, and 16.5% water (all in volume percent).
Portland cement is the primary binder used in conventional UHPC, but at a much higher proportion as compared to ordinary concrete or HPC. Cement with high proportions of tricalcium aluminate (C3A) and tricalcium silicate (C3S), and a lower Blaine fineness are desirable for conventional UHPC, as the C3A and C3S contribute to high early strength and the lower Blaine fineness reduces the water demand. The addition of silica fume fulfills several roles including particle packing, increasing flowability due to spherical nature, and pozzolanic reactivity (reaction with the weaker hydration product calcium hydroxide) leading to the production of additional calcium silicates. Quartz sand with a maximum diameter of about 600 μm is the largest constituent aside from the steel fibers. Both the ground quartz (about 10 μm) and quartz sand contribute to the optimized packing By reducing the amount of water necessary to produce a fluid mix, and therefore permeability, the polycarboxylate superplasticizer also contributes to improving workability and durability. Finally, the addition of steel fibers aids in preventing the propagation of microcracks and macrocracks and thereby limits crack width and permeability.
Despite performance advantages offered by UHPC, deployment has been slow. There are several possible reasons for this, including lack of a clear financial benefit to manufacturers. As would be expected, the costs of fabricating UHPC components are significantly higher than the costs of manufacturing conventional concrete components. Additionally, the higher cost of constituent materials in UHPC necessarily means that UHPC has a higher per-unit volume cost than conventional and high-performance concretes. Much of the cost of UHPC comes from its steel fiber, superplasticizer, and high purity fumed silica. Ultra-high performance fiber reinforced concrete is generally cured with heat and/or pressure to enhance its properties and to accelerate the hydration reaction of the binder, which also increases manufacturing cost.
The present invention relates to use of geopolymer composite (GC) binders, rather than Portland cement, for Ultra High Performance Concrete (GUHPC) applications.